Package cracking is a common problem in plastic encapsulated semiconductor devices. The problem arises from a combination of factors. One factor is an internal delamination between the plastic encapsulant material and a flag of a lead frame. The flag is a plate-like member of a conventional lead frame which supports the die. The flag, like the rest of the lead frame, is usually made of copper, a copper-alloy, or an iron-nickel alloy, and therefore has a coefficient of thermal expansion (CTE) which in most instances is different than that of the surrounding molding compound or plastic. As a result of this CTE mismatch, stress is created at the plastic-flag interface as the semiconductor device experiences temperature changes. The stress, upon reaching a maximum threshold, is relieved through delamination of the plastic-flag interface. Another factor involved in package cracking is moisture absorption. After the plastic-flag interface becomes delaminated, moisture from the environment diffuses through the molding compound to the delaminated area. Once moisture accumulates in the package, rapid temperature increases will cause the moisture to vaporize and expand, thereby creating an internal pressure pocket in the delaminated area. To relieve the pressure and associated stress, the surrounding plastic cracks. The most common occurrence of package cracking occurs when a user attaches a plastic semiconductor device to a substrate using a solder reflow operation. The high temperatures associated with solder reflow induce a rapid temperature increase which is often sufficient, depending on the moisture content of the device, to cause the plastic to crack.
There are numerous existing methods of dealing with the problem of package cracking. One method is dry-packing, which involves baking plastic encapsulated semiconductor devices sufficiently to reduce moisture content and packaging the devices into moisture resistant packets. Device users then attach the device before a sufficient amount of moisture to cause cracking can be absorbed into the package through exposure to ambient conditions. This method is effective, but significantly increases the costs of semiconductor devices. Moreover, device users must keep track of how long devices have been exposed to the ambient conditions to assure that any absorbed moisture is insufficient to cause cracking problems.
Other known approaches have tried to decrease the possibility of internal delamination through improved adhesion between flag and the plastic. For example, some manufacturers have "roughened" the metallic surface of the flag to improve adhesion. Others have formed small holes or dimples in the flag to provide anchoring mechanism. Another approach used to improve adhesion is use of a window-frame flag. A window-frame flag is more or less a hollowed-frame that supports the die, rather than a solid paddle.
The methods described above do, to some extent, decrease the possibility of delamination by providing better adhesion to the plastic encapsulating material. Similarly, the present invention has the advantage of reduced delamination, albeit through a technique not previously used. But in addition, the present invention addresses another manufacturing problem which the above methods do not address, namely the need for semiconductor manufacturers to use a different or customized lead frame design for each of its products. Having a different lead frame design for each die size and die functional pin-out necessitates a large inventory of piece-parts and requires additional time and human resources to design a new lead frame before each product introduction. In addition to costs associated with inventory and design, the cost of the lead frames themselves are undesirably inflated because of insufficient volume to lower the cost to a minimum. Several small orders for lead frames, each requiring separate tooling, is more costly than one order for the same total number of lead frames. Thus, the cost of semiconductor manufacturing could be significantly reduced by the use of a lead frame design which could be used in conjunction with several different die sizes. The present invention achieves this goal by utilizing a lead frame in a semiconductor device which can be used with various die sizes, while at the same time improving adhesion between the lead frame and the encapsulating material to alleviate package cracking problems.